ARM: rmobile: rcar-gen2: Activate bootm_size

Commit d245059f ("ARM: rmobile: rcar-gen3: Activate bootm_size")
only fixed the superfluous CONFIG_SYS_BOOTMAPSZ for R-Car Gen3, even
though it listed all affected boards. Apply the same fix to Gen2.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Fixes: d245059f ("ARM: rmobile: rcar-gen3: Activate bootm_size")
Cc: Eugeniu Rosca <erosca@de.adit-jv.com>